'Runaway' glaciers melting in Antarctica

A collapse of the giant West Antarctic ice sheet appears ''inevitable'', with only the timing of the melting and resulting sea-level rise of at least three metres uncertain, according to US scientists.

Unlike glaciers in Greenland and much of East Antarctica, the ice sheet in West Antarctic rests on bedrock that is below sea-level, leaving it exposed to melting as waters warm.

Two new papers, to be published this week in Science and Geophysical ResearchLetters, looked at the melting process under way on the region’s Thwaites Glacier and how the area's unique geography will contribute to a destabilising of the ice sheet as a whole.

Warming waters are bad news for major Antarctic ice sheets.

The melting of the glacier is already one of the biggest contributors to sea-level rise globally, shedding 52 gigatonnes of ice a year, or about half of West Antarctica’s losses.

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Thwaites Glacier will likely add less than a quarter of millimetre of sea-level rise a year over the next century but a rapid collapse would quadruple the rate and ''probably spill over into adjacent catchments, undermining much of West Antarctica'', scientists from the University of Washington’s Polar Science Centre said in their Science paper. Current sea-level rise is about three millimetres a year.

While modelling ''suggests a full-scale collapse of this sector may be inevitable, it leaves large uncertainty in the timing'', the researchers said. Full collapse may take 200-900 years.

Rapid melting on the Antarctic Peninsula. Photo: Nerilie Abram

The thinning ice is largely caused by warming offshore waters reaching under the ice floating at the continental margins. As reported in Fairfax Media on Monday, Australian researchers have found the band of westerly winds known as the Roaring Forties have quickened and moved closer to Antarctica, drawing up relatively warm waters from below the surface.

Even normally cold waters are around -1.5 to -1.8 degrees and warm enough to melt ice under pressure, said Tas van Ommen, a glaciologist with the Australian Antarctic Division.

Water now entering the continental shelf is ''hot'' by comparison, at 0.5-1.2 degrees. ''It is this intrusion that is the strong threat,'' Dr van Ommen said.

Exacerbating the problem is the area’s geography, with much of the ice sitting on bedrock that slopes inland, meaning the sheet gets thicker as the melt line moves closer to land.

''The thicker the ice gets, the higher the forces acting on it, and the more prone to flow that it becomes,'' Dr van Ommen said. ''Once you start the process, it runs away, it becomes unstable.''

Dr van Ommen agrees the study shows it is likely the ice melt is close to an irreversible threshold, but suggests that more sophisticated modelling and a better understanding of the warming ocean is needed. ''It’s right in the balance,'' he said.

However, work to link the ice research with projected climate change, including research under way at Australia’s Antarctic Climate & Ecosystems Cooperative Research Centre, may confirm whether the melting is unstoppable, and if the rates predicted by the US scientists are too fast or even too slow.

''With better, coupled modelling, we can say whether a metre or more in 200 years is realistic or not,'' Dr van Ommen said.